Conventional patient-matched instruments are provided with large surfaces that are configured to conform to a patient's unique anatomy. Successful surgical outcomes depend on the ability of patient-matched instruments to provide a reproducible, “confident” 3D-fit between the patient-matched instrument and the anatomy that they are designed to rest against. If there is any doubt by an end user that a patient-matched instrument fits well upon repeated engagement with a patient's unique anatomy, or if the instrument appears to fit well with the patient's anatomy in multiple spatial orientations with respect to the anatomy, the instrument is typically discarded, and the surgery is carried out with the use of conventional, non-patient specific instruments.
To date, at least some patient-matched surgical instruments for use in total knee arthroplasty have employed anatomy-contacting surfaces that are substantially “negatives” of distal femoral and proximal tibial articular joint surfaces. The anatomy-contacting surfaces are generally large surface areas that conform in a continuous manner to substantial areas of a patient's anatomy. In some instances, the custom surgical instruments are provided by obtaining 3D image data of the patient's anatomy (e.g., via an MRI scan), segmenting the 3D image data to clearly delineate surfaces of the bony and/or cartilaginous anatomy from surrounding tissues, converting the segmented data to a computer model via CAD or other software, performing one or more optional secondary processes (e.g., smoothing functions), using a computer model to customize one or more surfaces of an instrument to the patient's anatomy, and manufacturing the custom instrument such that it is adapted to conform to the patient's anatomy in a single spatial orientation.
In at least some current practices, substantially all portions of the joint anatomy shown in each 3D image data slice are segmented and conventional patient-matched instruments are provided with anatomy-contacting portions that contact substantially continuous areas of the patient's anatomy. Such anatomy-contacting portions have large continuous surface areas of contact with the patient's bone and cartilage, and therefore, it is critical that the engineers or automated programs creating the patient-matched instruments maintain a high level of accuracy and precision throughout each step of the entire segmentation process. Even if only one or two points on anatomy-contacting surfaces of a patient-matched instrument are inaccurate, misaligned, or otherwise misrepresent the true unique anatomy of the patient, the patient-matched instrument may not fit well, sit proud, teeter, wobble, or may not fit at all. In such instances, an end user is less likely to use the instrument. In many cases, poor patient-matched instrument fit may be attributed to even a few minor errors in the segmentation process.
Another drawback to using at least some conventional patient matched instruments is that smooth anatomy-contacting surfaces potentially allow the instruments to slide or slip when engaged with the patient's unique anatomy. For example, in some instances, body fluids in combination with slippery bone and cartilage may work against frictional forces between the instruments and anatomical portions. Moreover, due to the highly-conforming nature of conventional patient-matched instruments, improper seating may be exhibited when used with anatomy having an abundance of osteophytes, legions, or tumors. Lastly, soft tissue (e.g., fatty tissues) may gather between patient-matched blocks and bone or cartilage and create a false impression to a user that the instrument is seated properly, when it is in fact, the contrary.